The present invention relates to wireless digital communications, and more particularly to space-time diversity transmission systems and methods.
Spread spectrum wireless communications utilize a radio frequency bandwidth greater than the minimum bandwidth required for the transmitted data rate, but many users may simultaneously occupy the bandwidth. Each of the users has a pseudo-random code for “spreading” information to encode it and for “despreading” (by correlation) received spread spectrum signals and recovery of information. Such multiple access typically appears under the name of code division multiple access (CDMA). The pseudo-random code may be an orthogonal (Walsh) code, a pseudo-noise (PN) code, a Gold code, or combinations (modulo-2 additions) of such codes. After despreading the received signal at the correct time instant, the user recovers the corresponding information while other users' interfering signals appear noise-like. For example, the interim standard IS-95 for such CDMA communications employs channels of 1.25 MHz bandwidth and a pseudo-random code pulse (chip) interval Tc of 0.8138 microsecond with a transmitted symbol (bit) lasting 64 chips. The recent 3GPP wideband CDMA (WCDMA) proposal employs a 3.84 MHz bandwidth and the CDMA code length applied to each information symbol may vary from 4 chips to 256 chips. Indeed, UMTS (universal mobile telecommunications system) approach UTRA (UMTS terrestrial radio access) provides a spread spectrum cellular air interface with both FDD (frequency division duplex) and TDD (time division duplex) modes of operation. UTRA currently employs 10 ms duration frames partitioned into 15 time slots with each time slot consisting of 2560 chips (TC=0.26 microsecond).
The air interface leads to multipath reception, so a RAKE receiver has individual demodulators (fingers) tracking separate paths and combines the finger results to improve signal-to-noise ratio (SNR). The combining may use a method such as the maximal ratio combining (MRC) in which the individual detected signals in the fingers are synchronized and weighted according to their signal strengths or SNRs and summed to provide the decoding. That is, a RAKE receiver typically has a number of DLL or TDL code tracking loops together with control circuitry for assigning tracking units to the strongest received paths. Also, an antenna array could be used for directionality by phasing the combined signals from the antennas.
Further, UTRA allows for space-time block-coding-based transmit antenna diversity (STTD) in which, generically, channel bits b0, b1, b2, b3 (values ±1) are transmitted as the sequence b0, b1, b2, b3 by antenna 1 and simultaneously transmitted as the sequence −b2, b3, b0, −b1 by antenna 2. Note that interpreting (b0, b1) and (b2, b3) as two complex numbers (e.g., QPSK or QAM symbols) implies the sequence transmitted by antenna 2 differs from the sequence from antenna 1 by a rotation of π/2 in a two-complex-dimensional space followed by complex conjugation.
STTD is a diversity technique in the sense that it provides redundancy for the transmitted symbols. Recently, efforts in standardization of high speed downlink packet access (HSDPA) in WCDMA have taken place. The use of multiple antennas at the transmitter as well as the receiver is considered as a candidate technology. While STTD fits nicely in the grand scheme, STTD does not provide any increase in data rate relative to the current WCDMA systems.
Naguib et al, Increasing Data Rate over Wireless Channels, IEEE Signal Proc. Mag. 76 (May 2000) considers multiple antennas for both transmitter and receiver together with synchronous cochannel space-time coding (STTD) and interference cancellation. This approach can be adopted for HSDPA systems with 4 antennas at the base station, where each of the two pairs of antennas transmits one distinct data stream. This results in the transmission of two independent data streams, and thus doubles the system data rate as well as capacity. This scheme is thus double STTD (DSTTD) for a 4-antenna transmitter, or multiple STTD for a 2n-antenna transmitter with n>2.
For high data rate systems, the performance of a scheme can be evaluated based on the error rate (bit error rate, symbol error rate, frame error rate) as well as system throughput (the total data rate the system can support). While double/multiple STTD provides increased data rate and reasonably good performance, it can still be improved in some circumstances.
For practical wireless communication systems employing multiple antennas, small inter-element spacing is desirable. However, this often results in large correlation between the channel parameters especially when the channel angular spread is small. For high data rate schemes such as double/multiple STTD, high channel correlation results in significant performance loss due to the loss in diversity gain as well as decrease in the ability to separate multiple independent data streams.